Coated articles are known in the art for use in window applications such as insulating glass (IG) window units, vehicle windows, and/or the like. It is known that in certain instances, it is desirable to heat treat (e.g., thermally temper, heat bend and/or heat strengthen) such coated articles for purposes of tempering, bending, or the like in certain example instances.
In certain situations, designers of coated articles often strive for a combination of high visible transmission, substantially neutral color, low emissivity (or emittance), and low sheet resistance (RS). High visible transmission for example may permit coated articles to be more desirable in certain window applications, whereas low-emissivity (low-E) and low sheet resistance characteristics permit such coated articles to block significant amounts of IR radiation so as to reduce for example undesirable heating of vehicle or building interiors.
In applications such as vehicle windshields requiring a visible transmission of at least 70%, or even at least 75%, the coated article must be bent as well as heat treated. The bending is often performed by allowing a coated glass article to sag while being heat treated in a known manner. The heat treatment of such coated articles typically requires the use of temperature(s) of at least 580 degrees C., more preferably of at least about 600 degrees C. and often at least 620 degrees C., for about 5-10 minutes or more.
Unfortunately, heretofore, too much bending often causes such coated articles to suffer damage known as mottling. Mottling defects essentially result from cracking of the coating. With certain coated articles, mottling occurs when the coated article is heat bent to a significant extent. It is believed that the mottling damage results from high degrees of bending during heat treatment.
FIG. 1 is provided for the purpose of explaining the degree of bending of a coated glass article such as a vehicle windshield. FIG. 1 is a cross sectional view of a bent vehicle windshield. Parameter “x” in FIG. 1 represents the amount of bend in the windshield, and is known as a centerline convex value. The centerline convex value x is the distance between the apex of the interior surface of the windshield drawn straight down to a line (see the dotted line in FIG. 1) connecting the two ends of the windshield. This centerline convex value x is representative of the amount of bend (or depth of bend) in the windshield; the higher the value x, the higher the amount of bend in the windshield. FIG. 2 is a cross sectional view of a heat bent vehicle windshield (e.g., of FIG. 1, or an embodiment herein) which includes first and second heat bent glass substrates laminated to one another via a polymer inclusive layer (e.g., of or including PVB or any other suitable polymer inclusive material) 42, where the low-E (low emissivity) coating is provided on one of the substrates.
While many known windshields are capable of withstanding bends of about 18 mm (i.e., a centerline convex value x of about 18 mm), they cannot withstand bending to a greater extend without suffering from fatal mottling damage.
Consider the following coated article with the below-listed layer stack, where the layers are listed in order from the glass substrate outwardly.
Glass SubstrateTiO2Si3N4ZnOAgNiCrOxSnO2Si3N4ZnOAgNiCrOxSnO2Si3N4
While the aforesaid coated article is heat treatable, it cannot withstand significant degrees of heat bending without suffering fatal mottling damage. For example, such a coated article suffers fatal mottling damage at centerline convex values x of about 22-23 mm or more (i.e., a bend about 22-23 mm or more deep). In this regard, it is believed that the mottling damage begins at the interface between the silicon nitride and zinc oxide layers under the top silver layer, and then expands through the coating.
It will be appreciated by those skilled in the art that there sometimes exists a need for a vehicle windshield which is bent to a significant extent (e.g., bent to a centerline convex value x of at least about 24 mm, sometimes at least about 26 mm, or at least 28 mm, or even at least 30 mm in certain situations). Unfortunately, the coated article discussed above cannot be used in such applications because it cannot withstand such high degrees of bending without suffering fatal mottling damage.
Moreover, in vehicle windshield or other applications such as in insulating glass (IG) window units, lengthy heat treatments at high temperatures tend to cause the aforesaid coated article to suffer significant drops in visible transmission, significant changes in certain color value(s), and significant increases in sheet resistance (RS). Thus, there is room for improvement in one or more of these respects. Additionally, the aforesaid coated article is susceptible to scratching in certain instances, and is also sometimes characterized by high haze values following heat treatment in certain instances.
In view of the above, it will be apparent to those skilled in the art that there exists a need for coated articles which are capable of: (a) being bent to greater extents in applications such as vehicle windshields or the like, (b) being able to maintain acceptable optical characteristics when bent to such extents, (c) realizing improved or good thermal stability with regard to visible transmission, color, emissivity (or emittance), and/or sheet resistance (RS); (d) realizing improved mechanical durability such as scratch resistance; and/or (e) realizing improved haze characteristics. In certain example embodiments, it may be desired that one or more of these characteristics can be achieved.